Mgb2 Superconducting Wires: Basics And Applications

Basics and Applications

Ren?? Fl??kiger

• Publisher: WSPC
• Year: 2016
• eBook ISBN: 9789814725606
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

The compendium gives a complete overview of the properties of MgB 2 (Magnesium Diboride), a superconducting compound with a transition temperature of T c = 39K, from the fundamental properties to the fabrication of multifilamentary

Chapter 1

Vortex Matter in the Two-band Superconductor MgB₂

Leonardo Civale^*,‡ and Adriana Serquis^†

^*Los Alamos National Laboratory, Los Alamos, NM 87545, USA
^†Centro Atómico Bariloche, Bariloche 8400, Argentina
^‡[email protected]

The physics of vortex matter in MgB₂, the first discovered technologically relevant multiband superconductor (SC), are determined by the parameters of the two gaps and by their coupling. The interplay of the quasi-two-dimensional (2D) σ-band and the almost isotropic π-band results in anomalous and complex behavior of the characteristic lengths and critical fields. The coherence length and penetration depth are magnetic-field dependent, have different T dependence from those in single-band materials, and have different anisotropies. At low T and H the critical currents (J_c) are high and the relaxation rates (S) are low, indicative of large pinning energy. In clean MgB₂, as H and T increases, the π-band contribution is strongly suppressed, so J_c decreases and S increases sharply, but C-doping allows maintaining strong pinning up to high fields as needed for applications.

1.Introduction

The subject of this book is the superconducting compound MgB₂. Certainly, only a fraction of the hundreds of identified superconductors (SCs) have whole books dedicated to them. What is special about MgB₂?

Since the discovery of superconductivity in 1911 until 1973, the highest known transition temperature (T_c) increased from ~4K in Hg to ~23 K in Nb₃Ge, at a modest average rate of ~0.3 K/year.¹ By then the microscopic origin of superconductivity, based on electron–phonon interactions and described by the extremely successful BCS theory,² was well understood.³ The general consensus in the early 1980s was that this pairing mechanism was unlikely to produce significantly higher T_c values,⁴ so superconductivity research (at least from the fundamental science perspective) appeared close to a dead end. Everybody working in the condensed matter and material sciences communities at the time knows how this belief was dramatically shattered by Bednorz and Muller’s discovery in 1986.⁵ Not only did the highest T_c increase from ~30–164 K (HgBa₂Ca₂Cu₃O₈ under pressure) in 1993, at an average rate of ~20 K/year, but the proliferation of whole families of superconducting compounds opened up an unimagined richness of new physics, on top of the potential technological impact.¹

By 2001, the field had calmed down significantly, so the discovery of superconductivity⁶ in MgB₂, a simple and well-known binary compound, was a much welcomed surprise. It soon became clear that, in many regards, MgB₂ was more similar to conventional low T_c superconductors (LTS) than to the oxide SC (at that time simply called high temperature SCs, or High Temperature Superconductors (HTS)). For one thing, the observation of a significant boron (B) isotope effect⁷ indicated that superconductivity in MgB₂ was phonon–mediated, and strong experimental evidence confirmed the s-wave symmetry of the order parameter.^8–14 However, with T_c ~ 39 K, this clearly contradicted the old concept that the T_c of Nb₃Ge was close to the upper limit for this mechanism,⁴ indicating that MgB₂ was somehow unique and that it would force us to revise some previously accepted knowledge. The “uniqueness” of MgB₂ was reinforced by the fact that, in sharp contrast with the oxides, it was not just the first member of a large new family of SC. The initial hope that isostructural compounds with progressively higher T_c would soon be found did not materialize, only a few other borides and diborides turned out to be superconducting, but with much lower T_c.¹⁵ Chemical doping also proved very hard.¹⁵

Vortex physics had become a major research field in condensed matter and statistical mechanics since the discovery of the oxide HTS.¹⁶ When MgB₂ arrived, the vortex matter community knew from the beginning some of the basic questions to ask and had well-developed experimental and theoretical tools to address them, which was certainly not the case in 1986. Do grain boundaries behave as weak links and strongly limit inter-grain critical currents? How large is the anisotropy? Are there vortex liquid phases and melting lines? Are the solid vortex phases crystalline or glassy? What are the dominant pinning centers and mechanisms? Is the vortex dynamics Anderson–Kim (A–K) like or glassy? The previous experience from HTS helped a lot to make fast initial progress. Although overall vortex physics in MgB₂ turned out to be closer to LTS than to HTS, there was still another surprise: adding to its uniqueness, it was the first technologically relevant multiband SC.^8–14,17,18 As will be discussed below, this resulted in novel vortex behavior.

More recently, iron-based SC were discovered.¹⁹ Although they are multiband as MgB₂, important differences in the levels of hybridization soon became clear. In addition, they also constitute a huge new family, even larger than the oxides, and exhibit vortex dynamics as fast as the HTS,²⁰ again in sharp contrast with MgB₂.

2.Vortex Matter Concepts in Single-band SC

2.1Characteristic lengths and critical fields

In the simplest ...

Table of contents

• Cover
• Halftitle
• Series Editors
• Title
• Copyright
• Contents
• Foreword
• Preface
• List of Contributors
• 1. Vortex Matter in the Two-band Superconductor MgB2
• 2a. Synthesis, Substitutions and Properties of MgB2 Single Crystals
• 2b. Thin Film Deposition and Critical Fields
• 2c. Nanoscale Disorder in MgB2 Thin Films
• 2d. Structure-Property Correlation of MgB2 Wires and Tapes
• 3a. Structure and Properties of Bulk MgB2
• 3b. The Reactive Liquid Infiltration (RLI) Technique for the Bulk Reaction to MgB2
• 4a. Processing of Amorphous Nanosize Boron Powder
• 4b. MgB2 Wires Fabricated Using the Ex Situ Technique
• 4c. MgB2 Wires by In Situ Technique, Mechanical Alloying
• 4d. Pressure Effects on Jc of In Situ and Ex Situ Processed MgB2 Wires
• 4e. Fabrication of MgB2 Wires by Internal Mg Diffusion (IMD)
• 4f. Development and Properties of Advanced Internal Magnesium Infiltration (AIMI) Processed MgB2 Wires
• 5a. Material and Conductor Properties Relevant for Applications: A Fundamental Study
• 5b. AC Losses in MgB2 Wires
• 5c. Effect of Mechanical Load on Jc of MgB2 Wires
• 5d. Properties of Irradiated MgB2, Bulk and Wires
• 6a. MRI Magnets based on MgB2
• 6b. Bulk MgB2 Permanent Magnets
• 6c. Applications for very Fine MgB2 Wires
• 6d. MgB2 Transmission Lines for the Large Hadron Collider
• 6e. Hydrogen Cooled MgB2 Cables
• 6f. Wind Generator Projects based on MgB2 Superconductors